Malaria causes hundreds of thousands of deaths per year, but many adults in endemic regions are protected from severe symptoms by naturally acquired immunity (NAI). Passive transfer of malarial immunity confers protection, suggesting that a vaccine inducing immune responses similar to NAI could effectively stop malaria pathogenesis. While both T-cell and B-cell responses play a role in naturally acquired immunity to malaria, focusing the B-cell responses on conserved broadly-neutralizing functional epitopes significantly improves protection and may lead to sterile immunity. However, three aspects of parasite biology confound malaria vaccine development: (1) antigenic variability, (2) the presence of immunodominant but non-neutralizing epitopes in antigens, and (3) the diverse and numerous parasite antigens required for the three independent stages of the life cycle. These hurdles can now be overcome due to an explosion in technology for the structural definition of neutralizing epitopes in key malaria antigens. This work will ultimately inform the structure-guided design of immunogens for malaria vaccine development. We propose to reliably identify neutralizing-antibody epitopes and define the components required to elicit a strongly neutralizing immune response to malaria. These studies would form the basis for creating novel engineered immunogens that will harness the immune system to protect against P. falciparum and P. vivax the two species that cause the majority of malaria cases. In recent years, parasite surface proteins have been identified that are required for parasite viability and have the potential to elicit a neutralizing antibody response. While it is clear that antibodies targeting these surface proteins can reduce invasion, only a subset of antibodies that bind to these vaccine candidates are neutralizing, and an even smaller subset are broadly neutralizing against diverse strains of malaria. In addition, a complete halt of disease progression will require targeting multiple parasite proteins simultaneously, due to the functional redundancy within and across protein families available to the parasite. There is a significant gap in our understanding of the neutralizing potential of epitopes. In the absence of this knowledge, efficient vaccine design to prevent the pathogenesis of malaria will be severely hampered. In FY2019, we: 1) isolated and characterized naturally acquired human antibodies that target the P. vivax vaccine candidate the Duffy-binding protein (DBP) as described in scientific advances (Carias et al. 2019); 2) defined the structural basis for neutralization of P. vivax by DBP-specific naturally acquired human antibodies as described in scientific advances (Urusova et al. 2019); and 3) identified distinct neutralizing epitopes in the P. falciparum vaccine candidate erythrocyte binding antigen 140 (EBA-140) (Salinas et al. 2019). Our work has demonstrated that strongly-neutralizing antibodies recognize epitopes in functional regions of parasite proteins (for example oligomerization interfaces or receptor binding residues) rendering ligands non-functional, while non-neutralizing or weakly-neutralizing antibodies bind to non-functional regions. Thus, identifying strongly-neutralizing epitopes and eliminating weakly- or non-neutralizing epitopes is fundamental to designing an effective malaria vaccine. Once strongly-neutralizing epitopes have been identified these will be exploited for vaccine design, protein-based therapeutics and/or diagnostics.